But if the carrier tunes their network so you will never get a busy
signal when calling into 900 numbers from which they receive a
kickback (hosted on their network or just "preferred partners"), at
the cost of a greater likelihood of busy signals for calls which are
not as profitable for them, this is "enforcing the provider's
preferences and not the customers".
When carriers start to tune their network so not only do VOIP
connections to their own servers get a higher QoS, but also in a
manner which tends to *induce* jitter and other 'Q'uality degradation
for Skype and Vonage, then it's time for them to lose "common carrier"
protection.